Sialic acid acetyl esterase (SIAE) is an enzyme that removes acetyl moieties from the 9-OH position of sialic acid on N-glycans, making available ligands for some inhibitory Siglecs, the best studied member of this family being Siglec-2/CD22. The loss of SIAE results in enhanced B cell receptor signaling and a break in tolerance. Defective rare variants of SIAE have been discovered in human autoimmune subjects. SIAE appears to be an important regulator of a pathway of clonal ignorance in B cells, and the mechanism by which this clonal ignorance is maintained will be studied using B cell receptor knockin and transgenic models. Studies are also proposed to examine why the absence of SIAE results in an increase in CD4+ effector memory phenotype T cells. This phenotype might be mediated by a defect in B cells or a defect in dendritic cells or both. Studies will be performed to determine the mechanism that leads to this enhanced accumulation of effector memory phenotype CD4+ T cells. The potential for the functional categorization of conventional and plasmacytoid dendritic cells based on the level of 9-O-acetylation of sialic acid will be explored and the phenotype of a mutant mouse lacking a putative 9-O-acetyl transferase will be examined. Studies are also proposed to examine whether disease-related heterozygous catalytically defective variants of SIAE are misfolded proteins and a knockin approach will be used to ask if one of these variants functions in a dominant negative fashion in vivo. PUBLIC HEALTH RELEVANCE: Rare variants of the Sialic acid acetyl esterase gene have been discovered in autoimmune subjects and one function of this gene appears to be to prevent autoimmunity in mice and humans. The study of this enzyme and the pathway it regulates may provide both mechanistic insights about immunological tolerance and new therapeutic targets for autoimmune disorders.